In various fields (e.g., electronics, media, tool manufacturing, aircraft manufacturing, etc.), ion-enhanced processing is used for various kinds of workpiece treatment. The ion bombardment heats the workpiece and may cause microstructure and chemical element distribution profile changes, deformation, and even melting. This temperature is actively controlled.
As an example, ion-enhanced physical vapor deposition (IE-PVD) is widely used for coating various components (e.g., particularly for restoration of the airfoil profile of worn or damaged turbine blades). This technique is characterized either: by evaporation of a source material from a crucible by an electron beam; or by ion sputtering of a solid source material target and ion bombardment of condensate (deposited material) on the substrate (e.g., the blade). Ion bombardment provides enhanced adhesion and improved coating structure due to additional kinetic energy passed to ad-atoms. For ion bombardment, an ionizing discharge is formed in the vapor medium, and negative bias voltage is applied to the substrate (to accelerate positive ions extracted from the ionized vapor). To enhance process stability, the negative bias voltage is applied in a pulsed mode allowing control of duty cycle, pulse repetition frequency, and/or amplitude. See, US20040172826A1, Sep. 9, 2004 (U.S. '826). In that exemplary system, during the pre-deposition period (e.g., when loading a substrate holder with the workpiece(s), start heating, etc) and the coating process, the substrate can be moved (i.e., its location is not constant during coater setup and coating).
For formation of desired coating structure, it may be necessary to keep substrate temperature in a certain range during coating. With multicomponent alloy deposition, this range may be quite narrow. For example, U.S. '826 suggests that during deposition of two-phase Ti-6Al-4V substrate temperature should be kept in the range of 600-800° C.
Substrate temperature is affected by various factors: heat radiation from the melt pool in the crucible; substrate bombardment by high-energy electrons scattered by the melt pool; ion bombardment; and condensation of atoms out of vapor (latent condensation heat). As a result, substrate temperature can vary over a wide range, and substrate overheating may result. To avoid this, substrate temperature should be continuously measured in situ or simultaneously monitored and regulated in different ways, for example, by control of the duty cycle of bias voltage pulses.
In other situations, the temperature of a movable substrate (workpiece) may be measured with a thermocouple with wires brought out of the vacuum chamber through a sealed hollow sting shaft of the substrate holder. These wires are attached to the external measuring electric device through a rotating brush collector. See, for example, SU368349, Jan. 26, 1973 which involves deposition by evaporation. However, when the substrate is under high negative bias voltage (up to 10 kV) in an IE-PVD process, the direct connection of thermocouple wires to the grounded device is impractical.
Due to high electrical and magnetic fields in the ionizing plasma discharge there are additional problems involving electrical interference. The electrical interference may cause large temperature measuring errors in systems with high frequency or pulse power supply. Also, sparking and arcing on the workpiece can damage the measurement hardware.
Optical and electrically insulated thermocouple systems are used to avoid those problems.
Optical instruments for workpiece temperature measurement include pyrometers, laser interferometric systems, fluoro-optic thermometers, etc. Advantages of optical method are electrical isolation and avoiding electrical interference either due to a vacuum gap between the workpiece surface and parts of optical-electronic devices or due to a dielectric optic fiber connecting a temperature sensor and a reading system.
Pyrometers are often used for temperature measurement of substrates under high potential. For instance, an ordinary radiation pyrometer is disclosed in P. D. Parry, “Localized substrate heating during ion implantation”, J. Vac. Sci. Technol., 1978, vol. 15, No 1, pp. 111-115. An adaptively calibrated pyrometer is disclosed in B. I. Choi et al., “In situ substrate temperature measurement in high-Tc superconducting film deposition”, J. Vac. Sci. Technol., 1993, vol. All, No 6, pp. 3020-3025. Two-color or two-wave length infrared radiation is disclosed in: V. Korotchenko and A. Matthews, “Substrate temperature monitoring in plasma assisted processes”, Vacuum, vol. 36, No 1-3, pp. 61-65; K. Setsune et al., “Epitaxial Y—Ba—Cu—O thin films prepared by rf-magnetron sputtering”, J. Appl. Phys., vol. 64, No 3, pp. 1318-1322; and M. Inoue et al., “A two-pyrometer method of measuring substrate temperature”, J. Vac. Sci. Technol., 1991, vol. A9, No 6, pp. 3165-3168. Increase of accuracy of pyrometrical measurements is achieved by actually ascertaining the emittance of the body whose temperature is being measured. For this purpose the light reflection from the body is measured. See, for example, patents: Patton U.S. Pat. No. 5,029,117; Boebel et al. WO9429681; and Boebel et al. U.S. Pat. No. 5,564,830.
A laser interferometric temperature measurement system contains a glass transparent plate in vicinity of the substrate. Thermal increase of optical thickness of the plate caused by its thermal expansion due to heating by substrate radiation leads to changes in the resulting interferogram and allows calculation of the temperature (See, e.g., R. A. Bonds et al., “Temperature measurements of glass substrate during plasma etching”, J. Vac. Sci. Technol., 1980, vol. 18, No 2, pp. 335-338). The shadow effect provided by thermal expansion of the heated substrate is employed for temperature measuring in situ in Shirosaki JP9218104. Also, thermal deformation of substrate determined with help of a laser detector is used for substrate temperature monitoring in S. Inaba et al. JP2004303969.
Fluoro-optic temperature sensing systems have a phosphorescent probe (sensor) with temperature-dependent fluorescent decay time. The probe is placed near the substrate or attached to it and heated by heat radiation emitted by the substrate. A light pulse from flash lamp or LED is transmitted through an optic fiber to the sensor and excites the probe matter. After the light pulse is turned off, the decaying fluorescent signal continues to be transmitted through the fiber to the optic-electronic instrument, where a photosensitive detector registers the fluorescent signal. The measured decay time is then converted to a temperature value. See Wickersheim U.S. Pat. No. 4,560,286. Commercial fluoro-optic devices are available from the Luxtron unit of Luma Sense Technology, Inc. (Santa Clara, Calif.). The overall temperature range capability of this optical sensor technology is currently up to 330° C. Some variation of such an approach is presented in: Sato et al. JP4315935 and Ishikawa et al. U.S. Pat. No. 5,876,119. Examples employing such temperature sensors are described in E. G. Egerton et al., “Positive wafer temperature control to increase dry etch throughput and yield”, Solid State Technol., 1982, August, pp. 84-87; S. A. Shivashankar and B. Robinson, “Calorimetry in thin film processing”, J. Vac. Sci. Technol., 1986, vol. A4, No 3, pp. 1826-1829; and J. P. Blanchard, “Target temperature prediction for plasma source ion implantation”, J. Vac. Sci. Technol., 1994, vol. B12, No 2, pp. 910-917.
In Yaroslaysky U.S. Pat. No. 6,596,339 and U.S. Pat. No. 6,709,519, the temperature dependence of reflectivity of UV light on more than one spectral component is used for in situ temperature measuring of substrate film materials.
Method and apparatus utilizing the diffusion reflectivity of substrate surface and the dependence of the interband optical absorption edge on temperature are disclosed in Johnson et al. U.S. Pat. No. 5,388,909 and U.S. Pat. No. 5,568,978; Denton et al. WO0150109; Johnson et al. WO0233369 and US2004061057; Taylor et al. US20050106876A1. The method is applicable mainly to semiconducting materials.
WO2005052995 purports to enhance accuracy of temperature measurements with optical instruments by including in the measuring procedure creation of a temperature calibration curve for an optical instrument with help of thermocouple.
A general disadvantage of optical temperature measurement systems is sensitivity of the temperature reading to: 1) emissivity of substrate surface (so that after every change of substrate, a calibration with thermocouple is needed; also, the emissivity of the substrate continuously changes during coating; 2) relative location of substrate and optical device (sensor), that cannot be strictly fixed for systems with movable substrate holders; 3) narrow range of measured temperatures (except pyrometers); 4) harmful deposition of evaporated and other materials onto the optical sensors and windows, as well as parasitic sputtering of dielectric optical elements by ions that leads to large errors in the temperature measurement.
Also, processing equipment with optical temperature measurement devices is rather complex and expensive.
Application of electrically insulated thermocouples is described in X. Tian et al., “In situ temperature measurement in plasma immersion ion implantation”, Rev. Sci. Instr. 1999, vol. 70, No 6, pp. 2818-2821. A direct in situ temperature measurement technique employs a commercial K-type thermocouple attached directly to the workpiece. The thermocouple wire is insulated from the plasma and the grounded vacuum chamber wall through a quartz glass tube and a Teflon inlet at the chamber wall. The display meter is electrically floated together with the battery power supply. In Yukishige et al. JP 60 245 776, a sheathed thermocouple is inserted into a rotating substrate electrode under applied bias voltage to measure the electrode surface temperature and control the substrate temperature. Thermocouple leads go through a sealed hollow shaft to rotating brush collector contacts for connecting with external instruments. The thermoelectric method has three main advantages over the traditional use of a pyrometer and other optical sensors: 1) optical systems are calibrated for a certain temperature range, but the thermocouple approach can detect directly a wide range of temperatures; 2) pyrometers are usually set up outside of the chamber to aim at a certain point on the specimen, whereas the thermocouple may be attached in any suitable place; 3) the thermocouple unit is less costly.
However, that thermoelectric method and apparatus have disadvantages: 1) in safety precautions because the instruments cannot be touched when operating; 2) it may be needed to interrupt the ionizing discharge and application of the substrate bias voltage to get a good reading (because plasma process generates electrical noise and causes electrical interference, especially in pulse and RF modes of operation); 3) it is difficult to transmit measured signals to the grounded devices such as a signal processing unit, computer, and display; 4) the battery has limited service term that is especially significant for long deposition runs.
In R. W. Stitz et al., U.S. Pat. No. 4,632,056, a disk with attached thermocouple, located close to substrate and insulated from it, is used as a temperature sensor. Thermal radiation from the substrate affects the disk, so the thermocouple readings are related to the substrate temperature. However, this method is indirect and has low accuracy because of the influence of many disturbing factors, in particular due to variation of substrate surface radiation emissivity and disk surface absorptance. Also, accurate disk location relative to the substrate is needed which is difficult to achieve in a system with a movable substrate holder.